Process for producing a polyvinyl acetacetal resin for use in a heat transfer sheet

ABSTRACT

A heat transfer sheet comprises a substrate sheet and a heat transfer layer laminated on the substrate sheet and containing a dye which can be caused to migrate by heating to be transferred onto a transferable sheet, said heat transfer sheet containing a polyvinyl acetacetal resin as a binder component, the acetal moiety of said polyvinyl acetacetal resin being 50% by weight or more based on the total amount of the resin, and 80% by weight or more of said acetal moiety comprising polyvinyl acetacetal. 
     A process for producing a polyvinyl acetacetal resin by the reaction of a polyvinyl alcohol and acetaldehyde in an aqueous phase in the presence of 4 to 10% by weight of an acid catalyst comprises initiating precipitation of an acetalated product by maintaining the reaction system at a temperature of 8° to 17° C. for 30 minutes or longer, and then maintaining said reaction at a temperature of 25° to 40° C.

This is a division of application Ser. No. 07/132,001 filed Dec. 11,1987, now U.S. Pat. No. 4,902,670.

BACKGROUND OF THE INVENTION

This invention relates to a heat transfer sheet, and more particularlyto a heat transfer sheet which is particularly suitable for obtaining animage on a heat transferable sheet by carrying out heating printingcorresponding to image information by a thermal head or laser.

Further, the present invention relates to a process for producing apolyvinyl acetacetal resin, particularly to a process for producing apolyvinyl acetacetal resin of high acetalation degree and littleirregularity of particle size.

For obtaining an image corresponding to an information image by the useof a heating printing means such as thermal head or laser,heat-sensitive color forming has been primarily used in the prior art.In such heat-sensitive color forming paper, a leuco dye which iscolorless or pale yellow at room temperature provided on a substratepaper and a developer are brought into contact by heating to give acolor formed image. As such developers, phenolic compounds, zincsalicylate derivative, rosin, etc. have been generally employed.However, a heat-sensitive color forming paper has a serious defect ofcolor extinction when the color formed image is stored for a long term,and color printing is limited to two colors and cannot give a colorimage having continuous gradation.

On the other hand, a heat-sensitive transfer paper having a heat-fusiblewax layer comprising a pigment dispersed therein and provided on asubstrate paper has begun to be used in recent years. When theheat-sensitive transfer paper is superposed on a heat transferable paperand heating printing is carried out on the back surface of theheat-sensitive transfer paper, the wax layer containing the pigmentmigrates onto the transferable paper to produce an image thereon.According to such a printing method, by performing printing a pluralnumber of times by the use of a heat-sensitive transfer paper containingpigments of three primary colors, a multi-color image can be obtained,but it has been impossible to obtain a photograph-like image havingessentially continuous gradation.

In recent years, there has been an increasing demand for a technique toobtain photograph-like images directly from electrical signals, andvarious attempts have been made. One of such attempts is a method inwhich an image is formed on CRT, and this is photographed with a silversalt film. However, when the silver salt film is an instant film, thereis a drawback in that running cost becomes high, while, when the silversalt film is a 35 mm film, there is the drawback of lack of instancybecause a developing processing is required after photographing. Asstill another method, the impact ribbon method or the ink jet method hasbeen also proposed. However, the former involves the drawback of badimage quality, and the latter a drawback in that image cannot beobtained as simply as photography because image processing is required.

For solving such problems, there has been proposed a method in which aheat transfer sheet having a sublimatable disperse dye layer having theproperty of migration by heating provided thereon is used in combinationwith a heat transferable sheet, and the sublimatable dye is caused tomigrate under control onto the heat-transferable sheet, therebyobtaining an image having gradation like a photograph (Journal of ImageElectronic Society, Vol. 12, No. 1, 1983). According to this method, animage with continuous gradation can be obtained by simple processingfrom television signals, and yet the device used thereby is notcomplicated, whereby this method is now attracting attention.

As one of the prior art techniques approximating this method, the drytransfer printing method of polyester fibers may be mentioned. Thismethod is a method for obtaining an image, which comprises dispersing ordissolving a dye such as a sublimatable disperse dye, etc., in asynthetic resin solution to prepare a coating material, applying thecoating material in a pattern on a thin paper or like material, dryingto form a heat transfer sheet, superposing the heat transfer sheet ontoa polyester fiber which is the heat transferable sheet, and heating thecomposite under adhesion, thereby applying the disperse dye onto thepolyester fiber. However, even when the heat transfer sheet used in theprior art for the dry transfer printing method of polyester fibers isused as it is, and heating printing is carried out by a thermal head orthe like, it is difficult to obtain a color formed image with highdensity. The reason for this may be the fact that the heat-sensitivityof the heat transfer sheet is not high, and the dyeing ability of theheat transferable sheet is low.

Of these drawbacks, those caused by the heat transferable sheet werefound to be solved by a heat transferable sheet having a heattransferable layer comprising the island portions independent of eachother comprising a synthetic resin having a glass transition temperatureof -100° C. to 20° C. and a polar group and a sea portion comprising asynthetic resin having a glass transition temperature of 40° C. orhigher formed as an island-sea configuration (Japanese PatentApplication No. 135627/1983), but those caused by the heat transfersheet have not yet been solved. This is because, in the method ofprinting onto fibers, etc., migration and transfer of the dye areaccomplished by heating, for example, at 200° C. for about 1 minute,while heating with a thermal head is as short as several msec. at about400° C.

We have carried out various studies to obtain a heat transfer sheetwhich can be suitably used in combination with a heat transferablesheet, particularly a heat transferable sheet of Japanese PatentApplication No. 135627/1983 as mentioned above, in order to obtain animage of color photographic tone by heating printing with a thermalhead, etc., and consequently found the following facts.

In the heat transfer sheet generally used in the prior art, the dispersedye exists in a state wherein it is dispered as particles in a binder,and for sublimating the dye molecules under such a state by heating,heat energy breaking the interactions within the crystals and furthersurpassing the interaction with the binder must be imparted to the dyemolecules to accomplish sublimation and dyeing thereof onto the heattransferable sheet, whereby high energy is required. Also, in the casewhere, in order to obtain a colored image with high density, the dye iscontained in the binder resin at a high relative ratio, an image with asomewhat high density can be obtained. However, because of the weakenedbonding force in the heat transfer layer in the heat transfer sheet,when it is peeled off after printing with a thermal head with a heattransferable sheet superposed thereon, a phenomenon wherein the transferlayer is taken over together with the resin onto the heat transferablesheet is liable to occur. Further, since the dyes are of high cost, itis also disadvantageous to incorporate more dyes than necessary from thestandpoint of such intended purposes as OA instruments or home uses.

On the other hand, if it is possible to maintain dyes in a binder inmolecular dispersed form instead of particulate form, improvement ofheat sensitivity corresponding to the absence of the interaction withinthe crystals as in the case of particulate dispersion may be expected.However, even when such a state is merely attained within a binder, apractically useful transfer paper cannot be obtained. More specifically,thermally sublimatable dye molecules have relatively smaller molecularweights of about 150 to 550 and are mobile in the binder. Accordingly,for example, when a binder with a low glass transition temperature (Tg)is employed, there occurs the phenomenon wherein agglomeration occurswith elapse of time to cause precipitation, resulting ultimately in thestate of the dyes being dispersed in particulate form as describedabove, or due to bleeding onto the surface of the heat transfer layer,the dyes adhere also around the heating portion by the pressure betweenthe thermal head and the platen (pressurizing plate) during recording,whereby ground staining is generated to cause serious deterioration ofthe image quality.

Also, even if the glass transition temperature (Tg) of the binder ishigh, the dye molecules cannot be retained unless the molecular weightof the binder is large to some extent. Further, even when the dye isdissolved in molecular state in a binder with a high glass transitiontemperature (Tg) and somewhat large molecular weight, affinity betweenthe dye molecules and the binder is required in order to attain a statewhich is stable with time.

In view of such points, various heat transfer sheets intended to improveimage quality have been proposed. For example, as described in JapaneseLaid-Open Patent Publication No. 101087/1985, it has been known toobtain improvement of printing quality and stability with time by theuse of a specific polyvinyl butyral resin as the binder component in theink composition. However, these heat transfer sheets of the prior artare not necessarily amply satisfactory with respect to storability.

Generally speaking, as the conditions demanded for heat transfer sheet,storability of the heat transfer sheet itself is important along withvarious characteristics participating in image quality such as printingsensitivity and resolution. However, printing sensitivity andstorability greatly tend to cancel each other, and it is difficult toimprove both of these characteristics.

SUMMARY OF THE INVENTION

An object of the present invention, which has been accomplished in viewof the points as described above, is to provide a heat transfer sheetwhich is dramatically improved in storability without a lowering of itsprinting quality.

Another object of the present invention is to provide a process forproducing a polyvinyl acetal resin, herein specifically called"polyvinyl acetacetal" resin having excellent characteristics as aheat-resistant resin which is suitable, for example, as the bindercomponent to be added in the ink composition for the heat transfer sheetof the invention.

The heat transfer sheet according to the present invention comprises aheat transfer layer containing a dye which is caused to migrate byheating to be transferred onto a transfer receptor or transferable sheetlaminated on a substrate sheet, the heat transfer sheet containing apolyvinyl acetacetal resin as the binder component. Further, it ispreferable that the acetal moiety of the polyvinyl acetacetal resin be50% by weight or more based on the total amount of the polymer, and yet80% by weight or more of said acetal moiety should comprise polyvinylacetacetal. Also, the dye to be used is preferably a disperse dye and ispreferably in substantially dissolved state in the binder.

Furthermore, the present invention provides a process for producing apolyvinyl acetacetal resin by a reaction of a polyvinyl alcohol andacetaldehyde in an aqueous phase in the presence of 4 to 10% by weightof an acid catalyst, the polyvinyl acetacetal being highly suitable foruse as a binder component to be added in the ink composition for theheat transfer sheet of the invention. This process comprises initiatingprecipitation of acetalated product by maintaining the reaction systemat 8° to 17° C. for 30 minutes or longer, and then maintaining saidreaction at a temperature of 25° to 40° C.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIGS. 1 and 2 are fragmentary sectional views of examples of the heattransfer sheet according to this invention;

FIGS. 3 and 4 are perspective views of examples of the heat transfersheet of the invention; and

FIG. 5 is a schematic side view indicating an example of a method ofcarrying out transfer with a heat transfer sheet of the invention.

DETAILED DESCRIPTION OF THE INVENTION (I) Heat Transfer Sheet

The heat transfer sheet according to the present invention isconstituted of a heat transfer layer 3 provided on a substrate sheet 2as shown in FIG. 1.

Substrate Sheet

For the substrate sheet 2 to be used in the present invention, papers orfilms such as capacitor paper, polyester film, polystyrene film,polysulfone film, polyimide film, polyvinyl alcohol film, cellophane,aramide film, polyetherimide film, polyether ether ketone film,polybarbatic acid, etc., are used, and its thickness is 1.5 to 50 μm,preferably 2 to 9 μm. Among these papers or films, when low cost andheat resistance under untreated state are required, capacitor paper isemployed. On the other hand, when handling during preparation or runningin a thermal printer without breaking due to its high mechanicalstrength or surface smoothness is regarded as more important, apolyester film is preferably used.

Heat Transfer Layer

The heat transfer layer 3 comprises primarily a dye and a binder.

The dye is melted, diffused or sublimated by heat to be migratable.Particularly a disperse dye is preferably employed. These dyes havemolecular weights of about 150 to 550, and are selected withconsideration of the sublimation (melting) temperature, hue, lightresistance, solubility in ink and binder resin. In general,diarylmethane type, triarylmethane type, thizole type, methine type,azomethine type, xanthine type, oxazine type, thiazine type, azine type,acridine type, azo type, spirodipyrane type, indolinospiropyrane type,fluorane type, rhodaminelactam type, anthraquinone type, etc., arerepresentative dyes. More specifically, the following dyes arepreferable.

C.I. (Color Index) disperse yellow 51, 3, 54, 79, 23, 7, 141, 201;

C.I. disperse blue 24, 56, 14, 301, 334, 165, 19, 72, 87, 287, 154, 26;

C.I. disperse red 135, 146, 59, 1, 73, 60, 167;

C.I. disperse violet 4, 13, 26, 36, 56, 31;

C.I. solvent violet 13; CI. solvent black 3; C.I. solvent green 3;

C.I. solvent yellow 56, 14, 16, 29;

C.I. solvent blue 70, 35, 63, 36, 50, 49, 111, 105, 97, 11;

C.I. solvent red 135, 81, 18, 25, 19, 23, 24, 143, 146.

Specific examples are methine (cyanine) type basic dyes of monomethinetype, dimethine type, and trimethine type, such as3,3'-diethyloxathiacyanine iodide, Astrazone Pink FG (produced by BayerCo., C.I. 48015), 2,2'-carbocyanine (C.I. 808), Astrafiloxine (C.I.48070), Astrazone Yellow 7GLL (C.I. basic yellow 21), Aizen Catilonyellow 7GLL (produced by Hodogaya Kagaku, C.I. 48055), and Eizen CatilonRed 6BH (C.I. 48020); diphenylmethane type basic dyes such as Auramine(C.I. 655); triphenylmethane type basic dyes such as Malachite Green(C.I. 42000), Brilliant Green (C.I. 42040), Magenta (C.I. 42510), MethylViolet (C.I. 42535), Crystal Violet (C.I. 42555), Methyl Green (C.I.684), and Victoril Blue B (C.I. 44045); xanthene type basic dyes such asBilonin G (C.I. 739), Rhodamine B (C.I. 45170), and Rhodamine 6G (C.I.45160); acridine type basic dyes such as Acridine Yellow G (C.I. 785),Leonin Al (C.I. 46075), Benzoflavin (C.I. 791), and Afin (C.I. 46045);quinoneimine type basic dyes such as Neutral Red (C.I. 50040),Asthrazone Blue BGE/x125% (C.I. 51005), and Methylene Blue (C.I. 52015);and otherwise anthraquinone type basic dyes having quaternary amine.These dyes can be used in the forms as they are or in the forms obtainedby subjecting these dyes to alkali treatment, or alternativelycounter-ion exchanged derivatives or leuco derivatives of these dyes canbe also used. When a leuco dye, etc. which is colorless or pale coloredunder normal state is used, a developer is included in the transferablesheet.

Also, it is important that the dye be substantially dissolved in thebinder resin.

A specific feature of the present invention is the use of a specificpolyvinyl acetacetal resin as the binder resin. This polyvinylacetacetal resin can be obtained by acetalation of a polyvinyl alcoholand can be represented generally by the following repeating unitformula. ##STR1##

When a polyvinyl alcohol (PVA) is subjected to acetalation, as shown inthe above formula, it is difficult to achieve complete acetalation ofthe PVA, and acetyl groups or hydroxyl groups will inevitably remainpartially. As a result of our study, it has been clarified that resinshaving an acetalated moiety within a specific quantitative range areparticularly excellent in both storability and printing characteristics.More specifically, the polyvinyl acetacetal resin as the binder resinshould have 50% or more, preferably 62% or more, more preferably 70% ormore of the acetal moiety based on the total amount of the polymer, andyet 80% by weight or more, preferably 90% by weight or more of theacetal moiety should be polyvinyl acetacetal, for improvement ofstorability as well as printing characteristics.

If the above polyvinyl acetacetal resin has less than 50% by weight ofacetal moiety based on the total amount of the polymer, or (and) theamount of said acetal moiety in excess of 20% by weight comprise othercomponents than polyvinyl acetacetal, solubility in a solvent which candissolve well dyes such as toluene and MEK is lowered, whereby inkformation may become impossible in some cases. Also, since the glasstransition temperature correlated intimately with storability of heattransfer sheet is low, the printing density after storage is also lower,whereby there is also the drawback that precipitation of the dye may beobserved.

As the aldehyde to be used in the acetalation reaction in obtaining theresin as described above, acetaldehyde is generally employed, but forthe purpose of improving affinity of the substrate sheet for the binderresin or solubility of the binder resin in the solvent or reducing theresidual solvent, other aldehydes may be also used in an amount of 20%or less in terms of weight ratio at the acetalated portion. Examples ofthe aldehydes used for such purpose are formaldehyde, propionaldehyde,butylaldehyde, hexylaldehyde, 2-ethylhexylaldehyde, and the like, butthe present invention is not limited to these.

Further, the amount of the acetyl groups remaining in the polyvinylacetacetal resin has no essential influence on the accomplishing of theobjects of the present invention but it can be selected as desiredwithin the scope of the present invention as described above.

On the other hand, the molecular weight of the polyvinyl acetacetalresin can be suitably selected for the purpose of improving variouscharacteristics. For example, for the purpose of controlling theviscosity of the dye or improving the printing adequacy, resins obtainedfrom polyvinyl alcohols with different polymerization degrees byperforming separately acetalation reactions respectively may be mixed,and also a mixture of polyvinyl alcohols with different polymerizationdegrees at the stage of the starting material may be used.

The proportion of the dye to be contained in the heat transfer layer,which may also depend on the sublimation (melting) temperature of thedye and the magnitude of covering power under the color formed state, ispreferably 0.3 or more in terms of the weight ratio of the dye relativeto the above binder (dye/binder ratio), more preferably 0.3 to 3.0, mostpreferably 0.55 to 2.5. If the dye/binder ratio is less than 0.3, it isnot desirable in image quality such as printing density and heatsensitivity. On the other hand, with a ratio over 3.0, adhesion to thefilm and storability tend to be lowered.

Also, the binder resin may be also substituted by a cellulose type resinup to 10% by weight of the binder resin in the sense of improving thedrying characteristic when forming the heat transfer layer by coating.Examples of the cellulose type resin are ethyl cellulose, hydroxyethylcellulose, ethylhydroxy cellulose, ethylhydroxyethyl cellulose,hydroxypropyl cellulose, nitrocellulose and the like.

For providing the heat transfer layer on the substrate 2, a dye and abinder are dissolved together with a solvent to provide an inkcomposition for formation of the heat transfer layer, and this isprovided on a substrate 2 by a suitable printing method or coatingmethod. If necessary, any desired additive may be added in the inkcomposition for formation of the heat transfer layer.

The heat transfer sheet is basically constituted as described above, butwhen the surface of the substrate sheet is directly heated with acontact type heating means such as a thermal head, as shown in FIG. 2,by providing a lubricating layer 4 containing a lubricant or a moldrelease agent such as wax on the side of the support 2 where no heattransfer layer is provided, fusion between the heating means such as athermal head and the substrate can be prevented, and also slidabilitycan be improved.

The heat transfer sheet may be in the form of sheets cut into desireddimensions, or in a continuous or wind-up shape, or further in shape ofa tape with narrower width.

In providing the heat transfer layer 3 on the substrate sheet 2, acoating composition for heat transfer layer containing the same colorantmay be coated onto the whole surface of the substrate 2, but in somecases, a plurality of ink compositions for heat transfer containingdifferent colorants may be formed respectively on the different areas onthe surface of the substrate sheet, respectively. For example, a heattransfer sheet having a black heat transfer layer 5 and a red heattransfer layer 6 laminated in parallel on the substrate as shown in FIG.3, or a heat transfer sheet having a yellow heat transfer layer 7, a redheat transfer layer 8, a blue heat transfer layer 9 and a black heattransfer layer 10 provided repeatedly on the substrate sheet 2 can beused. By the use of a heat transfer sheet provided with plural heattransfer layers with different hues, a multi-color image can beadvantageously obtained with one heat transfer sheet.

On the transfer sheet perforations can also be formed or discriminationmarks can be provided for detecting the positions of the areas withdifferent hues for the purpose of convenience during usage.

Heat Transfer Method

The heat transfer sheet and the heat transferable sheet as preparedabove are so superposed that the heat transfer layer 3 on the heattransfer sheet 1 will contact the receiving layer 13 on the substratesheet 12 of the heat transferable sheet, and by imparting heat energycorresponding to the image information to the interface between the heattransfer layer and the receiving layer, the dye in the heat transferlayer migrates to the receiving layer.

As the heat source for applying heat energy, other than the thermal head14, known heat sources such as a laser beam, IR-ray flash, and thermalpen can be used. As the method for applying heat energy, it may beapplied from the heat transfer sheet side, or otherwise from the heattransferable sheet side, or both sides, but it is preferably appliedfrom the heat transfer sheet side from the standpoint of effectiveutilization of heat energy. However, it is more preferable to apply heatenergy from the heat transferable sheet side for better control of theheat energy applied, thereby expressing the gradation of the density ofthe image, or to promote diffusion of the colorant on the heattransferable sheet, thereby effecting expression of continuous gradationof the image more positively. Also the method of applying heat energyfrom both sides affords the advantages of the above two methods at thesame time.

When a thermal head is used as the heat source for imparting heatenergy, by modulating the voltage or the pulse width applied to thethermal head, the heat energy being applied can be varied continuouslyor in multiple steps.

When a laser beam is used as the heat source for applying heat energy,the heat energy can be varied by varying the dose of or irradiation areaof the laser beam. By the use of a dot generator having acoustic opticalelements built therein, heat energy can be also obtained depending onthe size of dots. Also, when employing a laser beam, it is preferablethat the heat transfer sheet adhere well to the heat transferable sheet,and the surface to be irradiated by the laser beam is preferably coloredblack for better absorption of the laser beam.

Alternatively, a substance which is nonsublimatable and can absorb thelaser beam to convert it to heat may be added in the heat transfer layer3, whereby heat can be transmitted to the dye more efficiently, and thedissolving ability can be enhanced.

When an IR-ray flash lamp is used as the heat source for imparting heatenergy, it may be used similarly as in the case of using a laser beam,or by the use of a pattern expressing continuously the density of theimage such as in black color or dot pattern, or the light may beprojected through these patterns. Alternatively, the colored layer ofone surface such as black color may be combined with a negative patterncorresponding to the negative of the above mentioned pattern.

By imparting heat energy to the interface between the heat transferlayer and the receiving layer as described above, the dye in the heattransfer layer is caused to thermally migrate to the receiving layer 13in the amount corresponding to the heat energy received thereat.

According to the heat transfer recording as described above, the dyecorresponding to the heat energy can be heat transferred to thereceiving layer to accomplish recording of one color. Furthermore, theabove method can be practiced by exchanging the heat transfer sheet, forexample, by exchanging successively with the heat transfer sheets ofyellow color, red color, blue color and, if necessary, black color, andheat transfer corresponding to the respective colors can be carried out,whereby a color image of color photographic tone comprisinghybridization of the respective colors can be also obtained. Instead ofusing the heat transfer sheets of the respective colors, by the use ofthe heat transfer sheets having areas formed previously by coatingseparately into the respective colors as shown in FIG. 4, first by useof the area of yellow color, yellow part image is heat transferred, thenheat transfer is carried out by the use of the area of red color,followed successively by repetitions of the same procedure with othercolors, whereby partial color images of yellow, red, blue and, ifnecessary, black colors can be heat transferred. By this method, thereis afforded an advantage in that exchange of the heat transfer sheet isnot required.

Also, by controlling the size of the heat source used for imparting theheat energy, the adhesiveness between the heat transfer sheet and theheat tansferable sheet, and the heat energy suitably, the image obtainedcan be improved in quality.

The heat transfer sheet according to the present invention can beutilized by combination with a heat transferable sheet for printing byuse of various printers of thermal printing systems, facsimiles, printpreparation of photographs according to the magnetic recording system,and print preparation from television screens.

For example, one television picture screen received can be memorized asthe signals of the respective partial color image patterns, the signalsof the respective partial color patterns outputted, the heat energiescorresponding to the signals imparted by means of a heat source asdescribed above such as a thermal head to the superposed heat transfersheet and heat transferable sheet, and heat transfer effectedsuccessively for the respective colors, whereby the television picturecan be reproduced as a print in the shape of a sheet. When a combinationof a heat transferable sheet with the heat transfer sheet of the presentinvention is utilized for print-out of such a picture, it is generallyconvenient for obtaining a reflected image to use as the heattransferable sheet a white receiving layer alone, or a colorlesstransparent receiving layer backed with a substrate such as paper or awhite receiving layer backed with a substrate such as paper.

The same operation as described above is also applicable when utilizinga combination of letters, figures, symbols and colors, etc., or graphicpatterns formed on CRT picture surface by an operation of a computer asthe original image, and also when the original image is a fixed imagesuch as a picture, photograph, printed matter, or a practical mattersuch as a person, stationary matter, or landscape, the above operationcan be carried out by the use of a suitable means such as a videocamera. Further, in creating the signals of the respective colors fromthe original image, an electronic plate making machine (color scanner)used for photographic plate making for printing may also be used.

(II) Process for Producing Polyvinyl Acetacetal Resin

Next, the process for producing a polyvinyl acetacetal resin havingparticularly excellent characteristics as the heat-resistant resin whichcan be used as the binder component in the ink composition for heattransfer layer of the above heat transfer sheet of the present inventionwill be described in detail.

Generally speaking, polyvinyl acetal resin has been known as a resin ofexcellent heat resistance. This resin can be obtained by condensationreaction of a polyvinyl alcohol with an aldehyde (formaldehyde,acetaldehyde, butylaldehyde, etc.). Particularly, as the number ofcarbon atoms forming the acetal ring of the polyvinyl acetal becomessmaller, the glass transition temperature becomes higher, whereby theheat resistance is better as is known in the art ("Mechanical Propertiesof Polymers", p. 19, published by Kagaku Dohjin, 1965). However,polyvinyl formal with the smallest carbon number of the acetal ring isspecific in solubility in solvents, and available solvents are limited.For example, polyvinyl formal with high formalization is soluble only inlimited solvents such as methylene chloride, methylenechloride-chloroform, methanol, glycol, formalin, furfural, andbenzene-alcohol. Therefore, the use of a polyvinyl acetacetal resin as aheat resistant resin has been desired.

In the condensation reaction in which a polyvinyl acetacetal resin isobtained by the condensation reaction of a polyvinyl alcohol withacetaldehyde, due to low reactivity of acetaldehyde, a highly acetalatedproduct (acetalation degree of 60 mol % or higher) cannot be easilyobtained. Polyvinyl acetacetal resins with low acetalation degree arewater-soluble, and become water-insoluble with the progress ofacetalation. For this reason, polyvinyl acetacetal resins are generallycommercially available as water-soluble acetals (low acetalationproducts). Such water-soluble polyvinyl acetacetal resins cannot be usedas the heat-resistant resin.

Even when the reactivity of acetaldehyde is enhanced by increasing thereaction temperature in order to obtain a polyvinyl acetacetal resinwith high acetalation degree, the dissolution limiting point (limitingacetalation degree which makes the polymer insoluble in water) of theacetalated product is lowered by increase in the reaction temperature tocause precipitation of low acetalation products. The low acetalationproducts are in the form of huge particles which cannot be easilyattacked by acetaldehyde, and therefore acetalation will not proceedfurther. Accordingly, no desired high acetalation product can beobtained. Besides, the huge particulate acetalation product also cannotbe readily purified because acid catalyst may remain with the particles,etc. Also, deviation in acetalation degree within the particles becomeslarger.

The process of the present invention solves the above problems of theprior art, and its object is to provide a process for producing apolyvinyl acetacetal resin with a high acetalation degree (60 mol % orhigher, preferably 65 mol % or higher, more preferably 70 mol % orhigher).

Another object of the present invention is to provide a process forproducing a polyvinyl acetacetal resin with little deviation in particlesize without huge particles. Still another object of the presentinvention is to provide a process for producing a polyvinyl acetacetalresin with little deviation in acetalation degree. Still another objectof the present invention is to provide a process for producing apolyvinyl acetacetal resin with no coloration. Still another object ofthe present invention is to provide a process for producing a polyvinylacetacetal resin with good solvent solubility.

The process of the present invention has been achieved on the basis ofthe discovery by the present inventors that, in a process for producinga polyvinyl acetacetal resin, by maintaining a high temperature afterprecipitation of a partially acetalated product by permittingacetalation to proceed gently at a low temperature in aqueous phase inthe presence of an acid catalyst, a polyvinyl acetacetal resin with highacetalation can be obtained, and that, by controlling the amount of theacid catalyst and the reaction temperature, a polyvinyl acetacetal resinwith little deviation in particle size, no coloration and also goodsolvent solubility can be obtained.

The process for producing a polyvinyl acetacetal resin of the presentinvention by the reaction of a polyvinyl alcohol and acetaldehyde inaqueous phase in the presence of 4 to 10% by weight of an acid catalystcomprises initiating precipitation of an acetalated product bymaintaining the reaction system at 8° to 17° C. for 30 minutes orlonger, and then maintaining said reaction system at a temperature of25° to 40° C., whereby the above objects can be accomplished.

Polyvinyl acetacetal has greater solubility in water as compared withpolyvinyl butyral. For example, while the dissolution limiting point(limiting acetalation degree which makes the polymer insoluble in water)is 20 to 25 mol %, the dissolution limiting point of polyvinylacetacetal is 45 to 60 mol %. The dissolution limiting point is loweredwith elevation in temperature. On the other hand, acetaldehyde has lowerreactivity with polyvinyl alcohol as compared with butylaldehyde.Therefore, in the production of a polyvinyl acetacetal resin, forobtaining a high acetalation product, it is necessary to carry out highacetalation by permitting acetalation to proceed gently in a statewherein polyvinyl acetacetal is dissolved in water. For such reasons, inthe production of polyvinyl acetacetal of the present invention, afteraddition of an acid catalyst and acetaldehyde to polyvinyl alcohol, thereaction system is maintained at a low temperature for a certain periodof time to accomplish gradually high acetalation and to precipitate theacetalated product. When precipitation of the acetalated product israpid, the dissolution limiting point is elevated by lowering thereaction temperature to ensure maintenance time of at least 30 minutesuntil precipitation initiation.

In the present invention, the method for adding acetaldehyde topolyvinyl alcohol is not limited to the specific one. The method for theadding step includes (a) one step addition, (b) divisional addition, and(c) continuous addition. In the case where said adding step (b) or (c)is used, it is necessary to maintain the reaction system, at least, at8° to 17° C. for 30 minutes or longer as mentioned above.

The amount of the acid catalyst is preferably in the range of 4 to 10%by weight. If it is lower than 4% by weight, ample acetalation reactioncannot proceed to produce polyvinyl acetacetal resin of a desiredacetalation degree. On account of a lower acetalation degree, theparticles will block each other to form huge particles. If it is over10% by weight, acetaldehyde undergoes aldol condensation due toexcessive acid, whereby there is the possibility of the polyvinylacetacetal resin becoming colored. As the acid catalyst, for example,hydrochloric acid, sulfuric acid, or nitric acid can be employed.

The amount of acetaldehyde is 0.7 to 2.2 mols, preferably 1.0 to 2.2mols, based on 2 mols (mols of hydroxyl groups) of polyvinyl alcohol. Ifit is lower than 0.7 mols, ample acetalation reaction cannot proceed toproduce polyvinyl acetacetal resin of a desired acetalation degree. Ifit is over 2.2 mols, the amount of acetaldehyde in the reaction systembecomes excessive, whereby the dissolution limiting point of theacetalated product will be increased. Accordingly, it takes a long timefor precipitation of the acetalated product, and besides irregularitiesin particle size occurs in the polyvinyl acetacetal resin obtained.

The reason is carried out in aqueous phase. In an alcohol such asmethanol, acetalation equilibrium will be established, whereby polyvinylacetacetal resin of high acetalation degree cannot be obtained.

The reaction system of polyvinyl alcohol and acetaldehyde in which anacid catalyst is added in aqueous phase is maintained at 8° to 17° C.for 30 minutes or longer, preferably 1 to 6 hours. By this procedure,acetalation proceeds gently to effect precipitation of the acetalatedproduct. If the temperature is lower than 8° C., precipitation of theacetalated product takes a long time, and besides irregularities inparticle size occur in the resultant polyvinyl acetacetal resin. If itis higher than 17° C., because the dissolution limiting point islowered, precipitation of a low acetalation product results. Theprecipitated low acetalation product is in the form of huge particles,which cannot be readily attacked by aldehyde, and therefore furtheracetalation will not proceed. For this reason, a polyvinyl acetacetalresin of high acetalation degree cannot be obtained.

The precipitated acetalated product has an average particle sizegenerally of 25 to 75 μm. If it is lower than 25 μm, scattering mayoccur during use of the resin, whereby workability will be lowered. Ifit is higher than 75 μm, in the subsequent maintenance at constanttemperature, further acetalation will not proceed to produce a desiredhigh acetalation product. Also, the acid catalyst may remain within theparticles to make purification difficult.

The reaction in which the acetalation product has been precipitated issubsequently maintained constantly at a temperature of 25° to 40° C.generally for 2 to 8 hours. If it is lower than 25° C., a highacetalation product may be obtained, but much low acetalation productsare contained, to make the distribution of the acetalation degree of thepolyvinyl acetacetal resin broader. If it is higher than 40° C.,acetaldehyde is volatilized into the gas phase portion of the reactionsystem, whereby acetalation, on the contrary, will be lowered. Byvolatilization of acetaldehyde, the environment around the reactionsystem will be contaminated. As caused by deacetalation or acetalformation between molecules, the solvent solubility of the polyvinylacetacetal resin obtained will be also lowered. Particularly,insolubilization in a nonalcoholic solvent (methyl ethyl ketone, etc.)will proceed.

The polyvinyl alcohol preferably has a polymerization degree of 500 to3500. If the polymerization degree of polyvinyl alcohol is lower, thereaction rate of acetalation becomes rapid. Therefore, even whenprecipitation of the acetalated product is accelerated by increasing thereaction temperature to lower the dissolution limiting point, a highacetalation product can be obtained. The residual acetyl groups in thepolyvinyl alcohol are suitably 0.5 to 12 mol %. The polyvinyl alcoholconcentration in the reaction system is not particularly limited, but itis generally made 4 to 10% by weight.

The present invention will now be described more fully with respect toExamples, which are presented as illustrative only and are not intendedto limit the scope of the present invention.

EXAMPLE A-1

A polyvinyl acetacetal resin to serve as the binder component wasproduced according to the following method.

First, a 5-liter separable flask was charged with 2,790 g of pure water,and 220 g of a polyvinyl alcohol (polymerization degree: 2,400, numberaverage molecular weight: about 135,000, saponification degree: 98.2%)was added thereto to be completely dissolved therein. Next, while theaqueous solution was maintained at a liquid temperature of 20° C., 650 gof 35% hydrochloric acid was added, and then the liquid temperature waslowered to 10° C., whereupon 137 g of acetaldehyde was added suitably toprecipitate colorless powder. Subsequently, the temperature of thereaction system was elevated to 30° C., maintained constantly thereatfor 3 hours, after which washing with water and neutralization werecarried out to remove the catalyst and unreacted aldehyde, thusobtaining a polyvinyl acetacetal resin. The polyvinyl acetacetal resinwas found to have an acetalation degree of 74.1 mol % (wt. %), 19.5% byweight of the vinyl alcohol moiety and a glass transition temperature(Tg) of 113° C.

EXAMPLE A-2

Example A-1 was repeated except for the use of a polyvinyl alcoholhaving a polymerization degree of 500, a number average molecular weightof about 30,000, a saponification degree of 98.2 mol %, to obtain aresin. The resin had the characteristics shown in Table 1.

EXAMPLE A-3

Example A-1 was repeated except for the use of a polyvinyl alcoholhaving a polymerization degree of 3,500, a number average molecularweight of about 200,000, a saponification degree of 98.2 mol %, toobtain a resin. The resin had the characteristics shown in Table 1.

EXAMPLE A-4

A resin was obtained in the same manner as in Example A-1 except for thechanging of the amount of aldehyde used to 80 g.

EXAMPLE A-5

Example A-1 was repeated except for the changing of the amount ofacetaldehyde used to 179 g, and, after precipitation of colorlesspowder, the elevating of the temperature of the reaction system to 35°C. and maintaining the system constantly at said temperature for 6hours.

EXAMPLE A-6

Example A-1 was repeated except for the use of a polyvinyl alcoholhaving a polymerization degree of 1,700, a number average molecularweight of about 100,000, a saponification degree of 99.6 mol %, toobtain a resin.

EXAMPLE A-7

Example A-1 was repeated except for the use of a polyvinyl alcoholhaving a polymerization degree of 1,700, a number average molecularweight of about 100,000, a saponification degree of 88.0 mol % and thechanging of the amount of acetaldehyde used to 111 g, to obtain a resin.

EXAMPLE A-8

A resin was obtained according to the procedure in Example A-1 exceptthat the reaction was carried out by adding suitably 10 g ofbutylaldehyde and 94 g of acetaldehyde as the aldehyde used.

EXAMPLE A-9

A resin was obtained according to the procedure in Example A-1 exceptthat the reaction was carried out by adding suitably 25 g ofoctylaldehyde and 80 g of acetaldehyde as the aldehyde used.

EXAMPLE A-10

A resin was obtained according to the procedure in Example A-1 exceptthat the reaction was carried out by adding suitably 20 g ofbutylaldehyde and 111 g of acetaldehyde as the aldehyde used.

EXAMPLE A-11

A resin was obtained according to the procedure in Example A-1 exceptfor the changing of the amount of aldehyde used to 63 g.

EXAMPLE A-12

Example A-1 was repeated except for the use of a polyvinyl alcoholhaving a polymerization degree of 2,400, a number average molecularweight of about 135,000 and a saponification degree of 88.0 mol %, thechanging of the amount of acetaldehyde used to 63 g, and, afterprecipitation of colorless powder, the elevating of the temperature ofthe reaction system to 35° C. and maintaining the system constantly atsaid temperature for 6 hours.

EXAMPLE A-13

Example A-1 was repeated except for the use of a aldehyde which wasobtained by adding 61 g of formaldehyde (35 wt. % solution of formalin)and 116 g of acetaldehyde. After colorless powder was precipitated, thetemperature of the reaction system was elevated to 40° C., and wasmaintained constantly at said temperature for 6 hours.

EXAMPLE A-14

Example A-1 was repeated except for the use of a polyvinyl alcoholhaving a polymerization degree of 1,000, a number average molecularweight of about 60,000, a saponification degree of 98.2 mol %, to obtaina resin. The resin had the characteristics shown in Table 1.

EXAMPLE A-15

Example A-1 was repeated except for the use of 24 g of a polyvinylalcohol (a polymerization degree of 500, a number average molecularweight of about 30,000, a saponification degree of 98.2 mol %) and 212 gof a polyvinyl alcohol (a polymerization degree of 2,400, a numberaverage molecular weight of about 135,000, a saponification degree of98.2 mol %), to obtain a resin. The resin had the characteristics shownin Table 1.

The characteristics of the resins obtained in the above Examples A-1 toA-15 are shown below in Table 1.

                                      TABLE 1                                     __________________________________________________________________________                                                Acetacetal                            Poly-                                                                             Number        Residual              relative to                           meriza-                                                                           average                                                                             Vinyl alcohol                                                                         acetyl                acetalated                        Exam-                                                                             tion                                                                              molecular                                                                           moiety  moiety                                                                              Acetalated moiety mol % (wt                                                                   moiety                                                                              Tg                          ple degree                                                                            weight                                                                              mol % (wt %)                                                                          mol (wt %)                                                                          Acet  Butyl                                                                              Octyl                                                                              wt %  (°C.)                __________________________________________________________________________    A-1 2400                                                                              ca. 135,000                                                                         24.1(19.5)                                                                            1.8(2.8)                                                                            74.1(77.7)                                                                          --   --   100.0 113                         A-2  500                                                                               ca. 30,000                                                                         22.4(18.0)                                                                            1.8(2.8)                                                                            75.8(79.2)                                                                          --   --   100.0 111                         A-3 3500                                                                              ca. 200,000                                                                         23.2(18.7)                                                                            1.8(2.8)                                                                            75.0(78.5)                                                                          --   --   100.0 112                         A-4 2400                                                                              ca. 135,000                                                                         33.2(27.5)                                                                            1.8(2.9)                                                                            65.0(69.6)                                                                          --   --   100.0 106                         A-5 2400                                                                              ca. 135,000                                                                         18.4(14.7)                                                                            1.8(2.8)                                                                            79.8(82.5)                                                                          --   --   100.0 122                         A-6 1700                                                                              ca. 100,000                                                                         26.6(21.8)                                                                            0.4(0.6)                                                                            73.0(77.6)                                                                          --   --   100.0 110                         A-7 1700                                                                              ca. 100,000                                                                         24.9(19.1)                                                                            12.0(18.0)                                                                          63.1(62.9)                                                                          --   --   100.0 103                         A-8 2400                                                                              ca. 135,000                                                                         24.2(19.4)                                                                            1.8(2.8)                                                                            69.0(71.4)                                                                          5.0(6.4)                                                                           --    91.8 104                         A-9 2400                                                                              ca. 135,000                                                                         29.2(23.2)                                                                            1.8(2.8)                                                                            65.0(66.9)                                                                          --   4.0(7.1)                                                                            90.4 100                          A-10                                                                             2400                                                                              ca. 135,000                                                                         24.2(19.1)                                                                            1.8(2.8)                                                                            64.0(65.4)                                                                          10.0(12.7)                                                                         --    83.8 100                          A-11                                                                             2400                                                                              ca. 135,000                                                                         48.2(41.4)                                                                            1.8(3.0)                                                                            50.0(55.6)                                                                          --   --   100.0  95                          A-12                                                                             2400                                                                              ca. 135,000                                                                         37.0(29.3)                                                                            12.0(18.5)                                                                          51.0(52.2)                                                                          --   --   100.0  92                          A-13                                                                             2400                                                                              ca. 135,000                                                                         26.9(22.3)                                                                            1.8(2.9)                                                                            56.3(14.2)                                                                          15.0(60.6)                                                                         --    81.1 116                          A-14                                                                             1000                                                                               ca. 60,000                                                                         24.3(19.7)                                                                            1.8(2.8)                                                                            73.9(77.5)                                                                          --   --   100.0 110                          A-15                                                                              500-1                                                                            ca. 127,000                                                                         23.9(19.4)                                                                            1.8(2.8)                                                                            74.3(77.8)                                                                          --   --   100.0 112                             2400-9                                                                    __________________________________________________________________________

COMPARATIVE EXAMPLE A-1

Example A-1 was repeated except for changing the amount of acetaldehydeused to 53 g.

COMPARATIVE EXAMPLE A-2

As the resin for binder, Denkabutyral "6000C" produced by Denki KagakuK.K. was used.

COMPARATIVE EXAMPLE A-3

A resin was obtained as in Example A-1 except that the reaction wascarried out by adding suitably 30 g of butylaldehyde and 111 g ofacetaldehyde as the aldehyde used.

The characteristics in the above Comparative Examples are shown in thefollowing Table.

                                      TABLE 2                                     __________________________________________________________________________                                                Acetacetal                        Compa-                                                                             Poly-                                                                             Number       Residual              relative to                       rative                                                                             meriza-                                                                           average                                                                             Vinyl alcohol                                                                        acetyl                acetalated                        Exam-                                                                              tion                                                                              molecular                                                                           moiety moiety                                                                              Acetalated moiety mol % (wt                                                                   moiety                                                                              Tg                          ple  degree                                                                            weight                                                                              mol (wt %)                                                                           mol (wt %)                                                                          Acet  Butyl                                                                              Octyl                                                                              wt %  (°C.)                __________________________________________________________________________    A-1  2400                                                                              ca. 135,000                                                                         54.2(47.2)                                                                           1.8(3.1)                                                                            44.0(49.7)                                                                          --   --   100.0 90                          A-2  --  ca. 155,000                                                                            (16.0)                                                                            --    --    --   --   --    90                          A-3  2400                                                                              ca. 135,000                                                                         22.6(17.6)                                                                           1.8(2.7)                                                                            61.2(61.6)                                                                          14.4(18.1)                                                                         --    76.8 87                          __________________________________________________________________________

EXAMPLE B-1

A 5-liter separable flask was charged with 2,790 g of pure water, and220 g of a polyvinyl alcohol (polymerization degree: 2,400,saponification degree: 98.8%) was added thereto to be completelydissolved therein. Next, while the aqueous solution was maintained at aliquid temperature of 20° C., 650 g of 35 wt. % conc. hydrochloric acidwas added. The amount of hydrochloric acid was 6% by weight. The liquidtemperature was lowered to 11° C., whereupon 143 g of acetaldehyde wasadded suitably to precipitate a colorless powder. The amount ofacetaldehyde was 1.3 mols per 2 mols of polyvinyl alcohol. The time fromaddition of acetaldehyde up to precipitation was 2 hours. The reactionsystem was elevated in temperature to 30° C. and maintained constantlythereat for 5 hours, after which washing with water and neutralizationwere carried out to remove the catalyst and unreacted aldehyde, thusobtaining a polyvinyl acetacetal resin. The polyvinyl acetacetal resinwas found to have an acetalation degree of 75.0 mol % (wt. %). Also, theresin had an average particle size of about 40 μm. These results areshown below in Table 3.

EXAMPLE B-2

A polyacetal resin was prepared as in Example B-1 except for changingacetaldehyde to 88 g (0.8 mol per 2 mols of polyvinyl alcohol). The timefrom addition of acetaldehyde up to precipitation was 2 hours. Thepolyvinyl acetacetal resin obtained was found to have an acetalationdegree of 69.3 mol % and an average particle size of about 40 μm. Theseresults are shown below in Table 3.

EXAMPLE B-3

A polyvinyl acetacetal resin was prepared as in Example B-1 except forusing 220 g of acetaldehyde (2.0 mols per 2 mols of polyvinyl alcohol),changing the reaction temperature to 10° C. and maintaining thetemperature after resin precipitation constantly at 35° C. for 5 hours.The time from addition of acetaldehyde up to precipitation was 3 hours.The polyvinyl acetacetal resin obtained was found to have an acetalationdegree of 77.1 mol % and an average particle size of about 40 μm. Theseresults are shown below in Table 3.

EXAMPLE B-4

A polyvinyl acetacetal resin was prepared as in Example B-1 except forchanging the reaction temperature to 9° C. and maintaining thetemperature after resin precipitation constantly at 35° C. for 5 hours.The time from addition of acetaldehyde up to precipitation was 6 hours.The polyvinyl acetacetal resin obtained was found to have an acetalationdegree of 75.0 mol % and an average particle size of about 25 μm. Theseresults are shown below in Table 3.

EXAMPLE B-5

A polyvinyl acetacetal resin was prepared as in Example B-1 except forchanging the reaction temperature to 16° C. and maintaining thetemperature after resin precipitation constantly at 35° C. for 2 hours.The time from addition of acetaldehyde up to precipitation was 30minutes. The polyvinyl acetacetal resin obtained was found to have anacetalation degree of 74.6 mol % and an average particle size of about75 μm. These results are shown below in Table 3.

EXAMPLE B-6

A polyvinyl acetacetal resin was prepared as in Example B-1 except forchanging the amount of hydrochloric acid to 420 g (4% by weight), thereaction temperature to 12° C. and maintaining the temperature afterresin precipitation constantly at 25° C. for 5 hours. The time fromaddition of acetaldehyde up to precipitation was 4.5 hours. Thepolyvinyl acetacetal resin obtained was found to have an acetalationdegree of 66.1 mol % and an average particle size of about 40 μm. Theseresults are shown below in Table 3.

EXAMPLE B-7

A polyvinyl acetacetal resin was prepared as in Example B-1 except forchanging the amount of pure water to 2,400 g, the amount of hydrochloricacid to 1,200 g (10% by weight), the amount of acetaldehyde to 220 g(2.0 mols per 2 mols of polyvinyl alcohol), the reaction temperature to9° C. and maintaining the temperature after resin precipitationconstantly at 40° C. for 8 hours. The time from addition of acetaldehydeup to precipitation was 1 hour. The polyvinyl acetacetal resin obtainedwas found to have an acetalation degree of 79.4 mol % and an averageparticle size of about 40 μm. These results are shown below in Table 3.

EXAMPLE B-8

A polyvinyl acetacetal resin was prepared as in Example B-1 except forusing a polyvinyl alcohol having a polymerization degree of 500, asaponification degree of 98.8 mol % and changing the amount ofhydrochloric acid to 420 g (4% by weight). The time from addition ofacetaldehyde up to precipitation was 1 hour. The polyvinyl acetacetalresin obtained was found to have an acetalation degree of 70.8 mol % andan average particle size of about 40 μm. These results are shown belowin Table 3.

EXAMPLE B-9

A polyvinyl acetacetal resin was prepared as in Example B-1 except forusing a polyvinyl alcohol having a polymerization degree of 3,500, asaponification degree of 98.8 mol %. The time from addition ofacetaldehyde up to precipitation was 5 hours. The polyvinyl acetacetalresin obtained was found to have an acetalation degree of 75.3 mol % andan average particle size of about 45 μm. These results are shown belowin Table 3.

EXAMPLE B-10

A polyvinyl acetacetal resin was prepared as in Example B-1 except forusing a polyvinyl alcohol having a polymerization degree of 2,400, asaponification degree of 88.0 mol %. The time from addition ofacetaldehyde up to precipitation was 2 hours. The polyvinyl acetacetalresin obtained was found to have an acetalation degree of 66.9 mol % andan average particle size of about 40 μm. These results are shown belowin Table 3.

EXAMPLE B-11

A polyvinyl acetacetal resin was prepared as in Example B-1 except forusing a polyvinyl alcohol having a polymerization degree of 1,700, asaponification degree of 99.2 mol % and changing the reactiontemperature to 10° C. The time from addition of acetaldehyde up toprecipitation was 3 hours. The polyvinyl acetacetal resin obtained wasfound to have an acetalation degree of 74.8% mol % and an averageparticle size of about 30 μm. These results are shown below in Table 3.

EXAMPLE B-12

A polyvinyl acetacetal resin was prepared as in Example B-1 except forusing 71.5 parts of aldehyde (0.65 mols per 2 mols of polyvinyl alcohol)and changing the reaction temperature to 16° C. The time from additionof acetaldehyde up to precipitation was 6 hours. The polyvinylacetacetal resin obtained was found to have an acetalation degree of62.5 mol % and an average particle size of about 195 μm. These resultsare shown below in Table 3.

EXAMPLE B-13

A polyvinyl acetacetal resin was prepared as in Example B-1 except forusing 264 parts of aldehyde (2.4 mols per 2 mols of polyvinyl alcohol)and changing the reaction temperature to 10° C. The time from additionof acetaldehyde up to precipitation was 9 hours. The polyvinylacetacetal resin obtained was found to have an acetalation degree of78.4 mol % and an average particle size of about 70 μm. These resultsare shown below in Table 3.

EXAMPLE B-14

A polyvinyl acetacetal resin was prepared as in Example B-1 except forusing a polyvinyl alcohol having a polymerization degree of 1,000, asaponification degree of 98.8 mol % and changing the amount ofhydrochloric acid to 420 g (4% by weight) and changing the reactiontemperature to 15° C. The time from addition of acetaldehyde up toprecipitation was 1.5 hours. The polyvinyl acetacetal resin obtained wasfound to have an acetalation degree of 70.6 mol % and an averageparticle size of about 40 μm. These results are shown below in Table 3.

EXAMPLE B-15

A polyvinyl acetacetal resin was prepared as in Example B-1 except forusing 24 g of a polyvinyl alcohol (a polymerization degree of 500, asaponification degree of 98.8 mol %) and 212 g of a polyvinyl alcohol (apolymerization degree of 2,400, a saponification degree of 98.8 mol %),and changing the reaction temperature to 12° C. The time from additionof acetaldehyde up to precipitation was 2 hours. The polyvinylacetacetal resin obtained was found to have an acetalation degree of74.6 mol % and an average particle size of about 40 μm. These resultsare shown below in Table 3.

COMPARATIVE EXAMPLE B-1

A polyvinyl acetacetal resin was prepared as in Example B-1 except forchanging the amount of hydrochloric acid to 200 g (2% by weight). Thetime from addition of acetaldehyde up to precipitation was 20 hours. Thepolyvinyl acetacetal resin obtained was found to have an acetalationdegree of 60.3 mol % and an average particle size of several mm. Thisresin was blocked. These results are shown below in Table 3.

COMPARATIVE EXAMPLE B-2

A polyvinyl acetacetal resin was prepared in the same manner as inExample B-1 except for changing the amount of pure water to 2,250 g, theamount of hydrochloric acid to 1,400 g (12% by weight) and changing thereaction temperature to 10° C. The time from addition of acetaldehyde upto precipitation was 30 minutes. The polyvinyl acetacetal resin obtainedwas found to have an acetalation degree of 77.2 mol % and an averageparticle size of about 40 μm. This resin was colored in pale yellow as awhole. These results are shown below in Table 3.

COMPARATIVE EXAMPLE B-3

Preparation of a polyvinyl acetacetal resin was attempted similarly asin Example B-1 except for changing the reaction temperature to 6° C.,but even after the elapse of 24 hours, no precipitation of the resin wasobserved. This may be considered to be due to the low reactiontemperature, whereby the dissolution limiting concentration of theacetalated product was too high.

COMPARATIVE EXAMPLE B-4

A polyvinyl acetacetal resin was prepared as in Example B-1 except forchanging the reaction temperature to 19° C. The time from addition ofacetaldehyde to precipitation was 6 minutes. The polyvinyl acetacetalresin obtained was found to have an acetalation degree of 70.0 mol % andan average particle size of about 400 μm. This resin was found to begreatly varied in particle size from 200 to 500 μm. These results areshown below in Table 3.

COMPARATIVE EXAMPLE B-5

A polyvinyl acetacetal resin was prepared as in Example B-1 except formaintaining the temperature after resin precipitation constantly at 50°C. for 3 hours. The time from addition of acetaldehyde up toprecipitation was 2 hours. The polyvinyl acetacetal resin obtained wasfound to have an acetalation degree of 71.5 mol % and an averageparticle size of about 60 μm.

The acetalation degree was lower than that of the resin obtained inExample B1. This may be considered to be due to volatilization ofacetaldehyde into the gas phase portion of the reaction system, wherebydeacetalation occurred. The polyvinyl acetacetal resin was insoluble innon-alcoholic solvent (e.g. methyl ethyl ketone, etc.). This may beestimated to be to insolubilization between molecules accompanied withdeacetalation. These results are shown below in Table 3.

COMPARATIVE EXAMPLE B-6

A polyvinyl acetacetal resin was prepared as in Example B-1 except forchanging the amount of hydrochloric acid to 1,408 g (13% by weight) andthe reaction temperature to 10° C. and maintaining the temperature afterresin precipitation constantly at 60° C. for 3 hours. The time fromaddition of acetaldehyde up to precipitation was 30 minutes. Thepolyvinyl acetacetal resin obtained was found to have an acetalationdegree of 68.4 mol % and an average particle size of about 70 μm.However, this resin was found to be colored pale yellow as a whole.

The acetalation degree was lower than that of the resin obtained inExample B1. This may be considered to be due to volatilization ofacetaldehyde into the gas phase portion of the reaction system, wherebydeacetalation occurred. The polyvinyl acetacetal resin was insoluble innon-alcoholic solvent (e.g. methyl ethyl ketone, etc.). This may beascribed to insolubilization between molecules accompanyingdeacetalation. These results are shown below in Table 3.

As is apparent from the above Examples and Comparative Examples,according to the process for producing polyvinyl acetacetal resin of thepresent invention, a polyvinyl acetacetal resin of high acetalationdegree can be obtained. This resin is little irregularity of particlesize and is also free from coloration. Furthermore, solvent solubilityis good. If the amount of hydrochloric acid is made 2% by weight, apolyvinyl acetacetal resin of high acetalation degree cannot beobtained. Due to low acetalation degree, the particles are susceptibleto blocking of each other to become huge particles. With an amount ofhydrochloric acid of 12% by weight, the resultant polyvinyl acetacetalresin will be colored pale yellow as a whole. If the reactiontemperature is 6° C., because the dissolution limiting concentration istoo high, no precipitation of resin can be observed even after theelapse of 24 hours. If the reaction temperature is made 19° C., theaverage particle size will become greater, and also the particle sizeswill vary widely. If the temperature constantly maintained during resinprecipitation is 50° C. or 60° C., the acetalation degree will belowered due to deacetalation. Moreover, the resin obtained becomesinsoluble in non-alcoholic solvent (e.g., methyl ethyl ketone).

                                      TABLE 3                                     __________________________________________________________________________    Polyvinyl    Acetalde-           Con- Con-                                    alcohol      hyde HCl  Reaction                                                                           Precipi-                                                                           stant                                                                              stant                                                                             Residual                                                                           Acetala-                                                                            Average                  polymeri-    charged                                                                            charged                                                                            tempera-                                                                           tation                                                                             temper-                                                                            temp.                                                                             acetyl                                                                             tion  particle                 zation       amount                                                                             amount                                                                             ture time ature                                                                              time                                                                              group                                                                              degree                                                                              size Other               degree       (mol)                                                                              (wt %)                                                                             (°C.)                                                                       (Hr) (°C.)                                                                       (Hr)                                                                              (mol %)                                                                            (mol %)                                                                             (μm)                                                                            quality             __________________________________________________________________________    Exam. B-1                                                                           2400   1.3  6    11   2.0  30   5   1.2  75.0  40   Excellent           Exam. B-2                                                                           2400   0.8  6    11   2.0  30   5   1.2  69.3  40   "                   Exam. B-3                                                                           2400   2.0  6    10   3.0  35   5   1.2  77.1  40   "                   Exam. B-4                                                                           2400   1.3  6     9   6.0  35   5   1.2  75.0  25   "                   Exam. B-5                                                                           2400   1.3  6    16   0.5  35   2   1.2  74.6  75   "                   Exam. B-6                                                                           2400   1.3  4    12   4.5  25   5   1/2  66.1  40   "                   Exam. B-7                                                                           2400   2.0  10    9   1.0  40   8   1.2  79.4  40   "                   Exam. B-8                                                                            500   1.3  4    16   1.0  30   5   1.2  70.8  40   "                   Exam. B-9                                                                           3500   1.3  6    11   5.0  30   5   1.2  75.3  45   "                   Exam. B-10                                                                          2400   1.3  6    11   2.0  30   5   12.0 66.9  40   "                   Exam. B-11                                                                          1700   1.3  6    10   3.0  30   5   0.8  74.8  30   "                   Exam. B-12                                                                          2400    0.65                                                                              6    16   6.0  30   5   1.2  62.5  195  "                   Exam. B-13                                                                          2400   2.4  4    10   9.0  30   5   1.2  78.4  70   "                   Exam. B-14                                                                          1000   1.3  4    15   1.5  30   5   1.2  70.4  40   --                  Exam. B-15                                                                           500-1 1.3  6    12   2    30   5   1.2  74.6  40   --                        2400-9                                                                  __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________    (bis)                                                                                Polyvinyl                                                                           Acetalde-           Con- Con-                                           alcohol                                                                             hyde HCl  Reaction                                                                           Precipi-                                                                           stant                                                                              stant                                                                             Residual                                                                           Acetala-                                                                           Average                          polymeri-                                                                           charged                                                                            charged                                                                            tempera-                                                                           tation                                                                             temper-                                                                            temp.                                                                             acetyl                                                                             tion particle                         zation                                                                              amount                                                                             amount                                                                             ture time ature                                                                              time                                                                              group                                                                              degree                                                                             size Other                       degree                                                                              (mol)                                                                              (wt %)                                                                             (°C.)                                                                       (Hr) (°C.)                                                                       (Hr)                                                                              (mol %)                                                                            (mol %)                                                                            (μm)                                                                            quality              __________________________________________________________________________    Comparative                                                                          2400  1.3  2    11   20.0 30   5   1.2  60.3 several                                                                            Blocked              Exam. B-1                                           mm                        Comparative                                                                          2400  1.3  12   10   0.5  30   5   1.2  77.2 40   Colored              Exam. B-2                                                                     Comparative                                                                          2400  1.3  6     6   --   --   --  --   --   --   --                   Exam. B-3                                                                     Comparative                                                                          2400  1.3  6    19   0.1  30   5   1.2  70.0 400  Blocked              Exam. B-4                                                                     Comparative                                                                          2400  1.3  6    11   2.0  50   3   1.2  71.5 60   Poor                 Exam. B-5                                                solubility           Comparative                                                                          2400  1.3  13   10   0.5  60   3   1.2  68.4 70   Poor                 Exam. B-6                                                coloration                                                                    &                                                                             solubility           __________________________________________________________________________

EXAMPLE C-1

On a polyethylene terephthalate film with a thickness of 6 μm (film forheat transfer, produced by Toray K.K.) applied on one surface withcorona treatment as the support, an ink composition for heat transferhaving the following composition was applied by wire bar coating to acoating amount of 1.3 g/m² on drying, which step was followed by drying,while the back surface was treated by applying one drop of a siliconeoil (X-41.4003A, produced by Shinetsu Silicone K.K.) and spreading itover the whole surface to provide a heat transfer sheet. As the binder,the resin obtained in Example A-1 was used.

    ______________________________________                                        Ink composition for heat transfer layer                                       ______________________________________                                        Dye (C.I. Solvent Blue 63)                                                                       4.5 wt. parts                                              Binder             4.3 wt. parts                                              Toluene            45 wt. parts                                               Methyl ethyl ketone                                                                              40 wt. parts                                               Isobutanol          5 wt. parts                                               ______________________________________                                    

The heat transfer layer of the heat transfer sheet thus obtained wastransparent, and no particles were visible when observed by a microscope(magnification×400).

Separately, by the use of a synthetic paper (Yupo FPG 150, produced byOji Yuka Goseishi K.K.), an ink composition for formation of a receivinglayer (image receiving layer) having the following composition wasapplied by wire bar coating to a thickness of 7 g/m² on drying and,after tentative drying by a hand dryer, dried in an oven at 100° C. forone hour to amply volatilize the solvent, thus providing a heattranferable sheet. By this drying, the amino-modified silicone oil andthe epoxy-modified silicone oil were bled partially to the surface toeffect crosslinking reaction, thereby forming a mold release layer.

    ______________________________________                                        Ink composition for forming receiving layer                                   ______________________________________                                        Polyester resin     10 wt. parts                                              (1:1 mixture of Vyron 200                                                     and Vyron 290 by Toyobo K.K.)                                                 Amino-modified silicone oil                                                                       0.125 wt. parts                                           (KF-393, produced by Shinetsu                                                 Silicone K.K.)                                                                Epoxy-modified silicone oil                                                                       0.125 wt. parts                                           (X-22-343, produced by Shinetsu                                               Silicone K.K.)                                                                Toluene             70 wt. parts                                              Methyl ethyl ketone 10 wt. parts                                              Cyclohexanone       20 wt. parts                                              ______________________________________                                    

The heat transfer sheet and the heat transferable sheet obtained asdescribed above were superposed so that the heat transfer layercontacted the receiving layer, and heat transfer was carried out by useof a heat transfer printer provided with a thermal head (partiallyglazed thin film type head, KMT-85-6MPD2, produced by Kyocera K.K.)under the following conditions.

Heat transfer conditions

Head application voltage: 11.25 V

Pulse width: 1.0-16.0 msec

printing speed: 33.3 msec/line

Density: 6 lines/mm

Platen hardness: 70°

Platen diameter: 25 mm

Line pressure: 4 kg/10 cm

Delivery speed: 5.0 mm/sec

After heat transfer, during peel-off of the heat transfer sheet from theheat transferable sheet, there was no transfer of the heat transferlayer, and there was no ground staining of the non-printed portionwhatsoever.

After heat transfer, the density at the site corresponding to thecurrent pulse width of 14 msec of the image formed on the heattransferable sheet was measured by a reflective densitometer (Macbethdensitometer RD-918).

Storability Test

A heat transfer sheet prepared in the manner described above except thatsilicone oil was not applied, and the same synthetic paper as the aboveheat transferable sheet were both cut into the same size, superposed sothat the heat transfer layer side of the heat transfer sheet contactedthe synthetic paper and then placed in superposed state in ahumidity-resistant packaging bag with a constitution ofPET/PE/aluminum/PE, sealed by heat sealing and thereafter placedstationarily in an oven at 60° C. under a pressure of 17 g/m² fromoutside of the packaging bag, to carry out a storability test.

Thereafter, the package was taken out from the oven, left to cool toroom temperature, and then the packaging bag was opened to take out theheat transfer sheet and the heat transferable sheet, and the state ofthe heat transfer layer of the heat transfer sheet was observed. As aresult, no change in appearance was detected.

After the storability test, the density of the dye transferred onto thesynthetic paper was measured by the reflective densitometer in the samemanner as described above.

Further, after the back treatment was applied on the back surface of theheat transfer sheet after the storability test according to the samemethod as described above, printing was carried out under the sameconditions as described above and the density of the image formed wasmeasured. As a result, similar printing densities as in the case ofusing the heat transfer sheet before the storability test could beobtained.

The above evaluations were also conducted by the use of heat transfersheets in which the dyes were changed to the following ones, and alsosimilar results could be obtained.

(1) Disperse Red 60

(2) Solvent Blue 36

(3) Solvent Blue 35

(4) Disperse Blue 14

(5) Disperse Blue 24

(6) Disperse Red 4

(7) Solvent Yellow 14

EXAMPLES C-12 TO C-12

Heat transfer sheets were prepared as in Example C-1 except for the useof the resins obtained in the above Examples A-2 to A-12 as the binder,and printing characteristics and storabilities were examined. Theresults are shown in Table 4.

EXAMPLES D-1 TO D-5

Heat transfer sheets were prepared as in Example C-1 except for the useof the following composition for the heat transfer layer.

    ______________________________________                                        D-1:                                                                          Dye (C.I. Solvent Blue 63)                                                                       4.5 wt. parts                                              Binder (A-1)       3.2 wt. parts                                              Binder (A-2)       0.3 wt. parts                                              Toluene            46.0 wt. parts                                             Methyl ethyl ketone                                                                              46.0 wt. parts                                             D-2:                                                                          Dye (C.I. Solvent Blue 63)                                                                       4.5 wt. parts                                              Binder (A-1)       2.85 wt. parts                                             Binder (A-2)       0.95 wt. parts                                             Toluene            45.85 wt. parts                                            Methyl ethyl ketone                                                                              45.85 wt. parts                                            D-3:                                                                          Dye (C.I. Solvent Blue 63)                                                                       4.5 wt. parts                                              Binder (A-1)       3.2 wt. parts                                              Binder (A-14)      0.3 wt. parts                                              Toluene            46.0 wt. parts                                             Methyl ethyl ketone                                                                              46.0 wt. parts                                             D-4:                                                                          Dye (No. 75199-13-2)                                                                             4.5 wt. parts                                              Binder (A-1)       3.0 wt. parts                                              Binder (BL-3, produced Sekisui                                                                   2.5 wt. parts                                              Kagaku-Kogyo, Japan)                                                          Toluene            45.75 wt. parts                                            Methyl ethyl ketone                                                                              45.75 wt. parts                                            D-5:                                                                          Dye (C.I. Solvent Blue 63)                                                                       4.5 wt. parts                                              Binder (A-15)      3.5 wt. parts                                              Toluene            46.0 wt. parts                                             Methyl ethyl ketone                                                                              46.0 wt. parts                                             ______________________________________                                    

In the following Tables (similarly in Table 5), "excellent" means thecase when the surface of the transfer layer became a glossy surface,while "good" means the case when no precipitation of dye occurred butthe surface became slightly matte.

                  TABLE 4                                                         ______________________________________                                                       Staining degree                                                Printing density                                                                             (Gray scale for                                                                           Appearance                                                Before  After   JIS staining                                                                            Before After                                 Example                                                                              storage storage fastness test)                                                                          storage                                                                              storage                               ______________________________________                                        C-1    2.22    2.20    4-5       Excellent                                                                            Excellent                             C-2    2.13    2.05    4-5       "      "                                     C-3    2.24    2.19    4-5       "      "                                     C-4    2.02    2.03    4-5       "      "                                     C-5    2.05    2.00    4-5       "      "                                     C-6    2.22    2.07    4-5       "      "                                     C-7    2.18    2.11    4-5       "      "                                     C-8    2.35    2.22    3-4       "      Good                                  C-9    2.41    2.25    3         "      "                                      C-10  2.16    1.98    3         "      "                                      C-11  2.35    2.20    3         Good   "                                      C-12  2.42    2.25    3         "      "                                     D-1    2.41    2.33    4         Excellent                                                                            Excellent                             D-2    2.38    2.27    4-5       "      "                                     D-3    2.35    2.22    4-5       "      "                                     D-4    2.43    2.37    4-5       "      "                                     D-5    2.40    2.33    4         "      "                                     ______________________________________                                    

COMPARATIVE EXAMPLES C-1, C-2 AND C-3

Heat-transfer sheets were prepared as in Example C-1 except for the useof the resins obtained in the above Comparative Examples A-1, A-2 andA-3 as the binder, and printing characteristics and storabilities wereexamined. The results are shown in Table 5.

                  TABLE 5                                                         ______________________________________                                                           Staining degree                                            Compar-                                                                              Printing density                                                                          (Gray scale for                                                                           Appearance                                     ative  Before  After   JIS staining                                                                            Before After                                 Example                                                                              storage storage fastness test)                                                                          storage                                                                              storage                               ______________________________________                                        1      2.35    1.64    2         Excellent                                                                            Dye                                                                           precipi-                                                                      tated                                 2      2.23    1.72    2         "      Dye                                                                           precipi-                                                                      tated                                 3      2.17    1.87    1.2       "      Dye                                                                           precipi-                                                                      tated                                 ______________________________________                                    

As is apparent from the results of the above Examples, since the heattransfer sheet of the present invention uses a specific polyvinylacetacetal resin as the binder component constituting the heat transferlayer, it has excellent image quality and storability, and dramaticallyimproved effects can be obtained particularly in storability for a longterm.

Also, in the production of the polyvinyl acetacetal resin of the presentinvention, since the reaction temperature and the constant temperaturemaintaining conditions are controlled in the presence of a constantamount of the catalyst, a polyvinyl acetacetal resin of high acetalationdegree can be obtained. This resin is free from huge particles, and withlittle deviation in particle size and without coloration. Therefore, itcan be easily purified. Further, solvent solubility is good, and it isalso soluble in a non-alcoholic solvent (e.g., methyl ethyl ketone). Inthe manufacturing steps, aldehyde will not be volatilized, whereby noproblem of environmental pollution can arise.

As a result, the polyvinyl acetacetal resin obtained by the process ofthe prevent invention is useful as a heat-resistant resin which ishighly suitable as a binder component added in the ink composition forthe heat transfer sheet of the invention.

What is claimed is:
 1. A process for producing a polyvinyl acetacetalresin, sequentially comprising the steps of:initiating precipitation ofan acetalated product by maintaining an emulsifier-free reaction systemcomprising a polyvinyl alcohol and acetaldehyde in an aqueous phase inthe presence of 4-10% by weight of an acid catalyst in a firsttemperature range of 8°-17° C. for at least 30 minutes; and maintainingthe reaction system in a second temperature range of 25°-40° C. toobtain the polyvinyl acetacetal resin.
 2. The process of claim 1,wherein said acid catalyst comprises at least one acid selected from thegroup consisting of hydrochloric acid, sulfuric acid, and nitric acid.3. The process of claim 1, wherein said acetaldehyde is provided for thereaction in a proportion of 0.7 to 2.2 mols per 2 mols of said polyvinylalcohol.
 4. The process of claim 1, wherein the acetalation degree ofsaid polyvinyl acetacetal resin is 60 mol % or higher.
 5. The process ofclaim 1, wherein the acetalation degree of said polyvinyl acetacetalresin is 65 mol % or higher.
 6. The process of claim 1, wherein theacetalation degree of said polyvinyl acetacetal resin is 70 mol % orhigher.